The present invention relates to a method of evaluating the peel strength of the plastic sliding layer of a laminate for sliding surface bearings, which comprises a metallic backing layer consisting preferably of steel, a sintered or sprayed-on, porous bronze layer having a thickness of 0.1 to 1.0 mm and containing a rolled-in pore filler, and on said bronze layer a sliding layer having a thickness of 0.005 to 1.0 mm and made of perfluoro polymers, particularly polytetrafluorethylene, which may contain additives for improving the emergency running properties, such as lead, graphite, molybdenum disulfide, individually or in combination, and, if desired, also additives for improving the wear resistance and the load-carrying capacity, such as glass fibers, carbon fibers, glass beads, individually or in combination.
Laminates for sliding surface bearings constitute a combination of a metallic backing layer and a plastic sliding layer. As a rule, a sintered or sprayed-on, porous bronze layer consisting of tin bronze or tin-lead bronze and having a thickness of 0.1 to 5.0 mm is provided between the sliding layer and the metallic backing layer. The filler in the pores of the bronze layer and the sliding layer provided in a thickness of 0.01 to 0.5 mm on the bronze layer may consist of polymers, preferably of polytetrafluoroethylene, which may contain friction-reducing additives, such as lead, molybdenum disulfide and/or graphite, in an amount of 5 to 40 wt. %. The wear resistance and the load-carrying capacity may be improved by an addition of glass fibers, carbon fibers and/or glass beads in an amount of 5 to 40%. The porous bronze layer ensures a very strong bond between the plastic used and the metallic backing layer. The bond to the metallic backing layer ensures that the sliding surface bearing elements made from such laminates will be highly similar to sliding surface bearing elements made of metallic laminates as regards interference fit, elastic properties and thermal expansion so that they can be used with good results even if they have only a small wall thickness. Compared to all-plastic materials for sliding surface bearings, laminates for sliding surface bearings comprising a plastic sliding layer have the advantage that they have a higher thermal conductivity and a higher strength. The sliding surface bearing elements made from such laminates generally have a high load-carrying capacity up to about 150 N/mm.sup.2.
Whereas, the laminates for sliding surface bearings of the kind described first hereinbefore were originally conceived for non-lubricated bearings, they are presently used also under conditions of mixed friction and with hydrodynamic lubrication owing to their excellent antifriction properties. For this reason they have a wide field of application. For instance, sliding surface elements made of such laminates are used in machines, motor vehicles, aircraft and apparatus and in electrical engineering. For this reason all those properties and features of laminates for sliding surface bearings which are required for bearings meeting specified requirements must be ensured by an extensive quality assurance program. One of the most important features which determine the quality of the laminate for sliding surface bearings is the peel strength of the plastic sliding layer on the porous bronze layer.
The peel strength of the plastic sliding layer has previously been evaluated by the cross hatch test used in accordance with DIN 53 151 for testing paints. In that test, a multiple-knife implement having six knife edges spaced 1 to 2 mm apart is used to form a sample of the laminate in a strip-shaped area with six incisions extending as far as to the bronze layer and in another strip-shaped area, which extends at right angles to the first, also with six of such incisions so that a grid of 25 squares is formed. After the grid has been made, a hand brush applied under a light pressure is reciprocated five times on the grid in both diagonal directions. The cross-hatched area is then inspected with a magnifying glass and is compared with illustrations and descriptions provided in an evaluating table so that a grid parameter is determined by which the peel strength of the sliding layer is defined. The cross hatch test has the disadvantage that errors in the evaluation cannot be avoided.